Tube tester



Feb. 20, 1940. F, E, WENGER TUBE TESTER Filed dot. 14,' 193s [NveNTOQ T /CZoYD E. WET/@EQ Patented Feb. 20, 1940 UnireN stares TUBE TESTER.

Floyd E. Wenger, Bluiiton, Ohio, assigner to Ray L. Triplett, Bluton, Ohio Application October 14, 1938, Serial No. 235,032

4 Claims.

The present invention relates to electrical testing apparatus and, in particular, tothe measurement of the dynamic coefficient of space current devices, such as electronic tubes employed for t radio `and similar purposes.

According to the'present day methods of measuring such coemcients, it is customary to connect the tube under test leither directly to a dynarnometer form o1" measuring instrument or indirectly H1 through a rectiiier, in which case the instrument is of the direct current or DArsonval type. In the case of the direct connection, it has been found that the alternating current ammeter is somewhat insensitive, because the energy for the moving l and stationary coils is abstracted and, therefore, seriously detracts from the output energy of the tube under test.

In the rectier form of connection, a copper oxide (Rectox) or a two-electrode tube is usually 20" employed. However, these devices are also affected by error in that the copper oxide layer may become de-activated and the thermionic device is subject to deleterious space charge effects. In addition, both forms of devices intro- 25 duce errors brought about by changes in temperature in load conditions. In all of these cases, it is difficult, iinot completely impossible, to accurately Inateh internal characteristics of the tube under test with the electrical characteris- 30 tics of the immediately associated circuit.

The primary object of the present `invention is to obviate these sources oi error and in general to improve on the present` day methods of, and apparatus for, determining the dynamic characteristics of a radio tube. This object is carried out in brief by interposing between the tube under test and the meter, a thermionic tube having a resonance cut-orf, and operating the interposed tube at its cut-off value, that is, with no signal 4 0 voltage on the grid, no current iiows in the plate circuit. This interposed tube will be called hereinafter the output tube.

Other objects and features will be apparent as the specication is p-erused in connection with 45 theaccompanying drawing, which shows a typical circuit for carrying out my invention.

Inthe drawing:

The figure shows a schematic diagram oi a circuit, which may be employed for determining 5U or measuring the dynamic coefficients of a radio tube in accordance with. the present invention.

, Referring to the drawing in detail, the tube `under test is designated by the reference char-- acter I and is exemplied as taking the form 55x. of a three-electrode device having a lainentary cathode 2, a grid 3 and a plate ll. For energizing these electrodes, a transformer 5 is employed, the primary winding E of which is connected to the A. C. mains indicated at 7. The secondary of this transformer is constituted of a number of coils 6 .oi which the lefthand coil 8, as shown, is connected through a variable tap indicated at 9 to the filament of the tube. This circuit is grounded at l0. The voltages for the grid and plate circuits of the tube l are obtained from a pair of 10 half-wave rectiiiers combined in a single bulb and designated by the reference character l I.

The elements of this double half-wave rectifier` are constituted of a iilament l2, a pair of cathodes i3, I i and a pair of anodes l5, it. The filament energy is obtained from a secondary coil Vl, and connections between the cathode i3 and the anode i6 are taken from opposite ends of a secondary coil i8, the mid-point of which is grounded. The plate l5 is connected to a 20 filter constituted of a choke coil i9 and a condenser shunted by a iiXed resistor 2l. A connection 22 is taken from a terminal between the choke and condenser to a potentiometer network consisting of two fixed resistors Z3 and 2t ar- 25 ranged in parallel and connected to` ground as at 25. The terminal between the condenser 220 and the resistor El is also grounded as at 2li.

There is a conductor 2l passing from an adinstable tap 20 on the resistor 2l through a con- 30 denser 25B to an `adjustalzxle tap 30 on the resistor 23. Aconnection .il is taken from the adjustable tap titi to the grid 3 of the tube under test and serves to apply a negative grid bias to the tube. the magnitude of which can be controlled by the $5 position oi the tap on the resistor 23.

The plate circuit of the tube i is connected through the primary coil Si of a transformer 33 to an adjustable tap 3ft on a resistor 35. One end of this resistor is connected to ground at Si, and the other end is connected through a filter consisting of a choke 3l and grounded condensers 3S to the cathodlll of the rectier il.

A consideration of the circuits explained up to this point will show that there is available "at the tap 30 a pulsating unidirectional potential of negative polarity with respect to ground which is applied or superposedon the grid t. The space current path between the anodel i5 and cathode i3 oi the device il recties the alternating volt- 50 age obtained from the righthand portion of the coil` lli, and this voltage is filtered through the elements it and 2t to remove or smooth out the current undulations to a large extent, but leaving a` small ripple which serves as a source of signal 55 voltage applied to the grid of tube I. This voltage is impressed across the two resistances it, 2st. The connections within the rectifier are such that the righthand ends (as shown) of the resistors 23, 24 represent terminals of positive potential with respect to ground so that the tap 3i! as well as the tap @Mathe function of which will be described hereinafter, pick oii negative voltagthe magnitude of which is determined by the position of the respective taps on their resistors.

On the other hand, the lefthand end (as shown) of the resistor 35 represents a terminal of positive potential, due to the manner in which the plate IS and cathode I4 of the rectifier I l arev connected to the transformer energizing circuit. As in the case of the vgrid voltage, the current through the resistor 35 is filtered by the choke-condenser network 3l, 38, to provide a voltage of rsubstantially constant magnitude. The voltage at the tap 34 is of positive polarity with respect to ground and is impressed on the plate l of the tube under test through the conductor 3Q and the transformer primary 32.

As stated hereinbefore, in carrying out the objects of the present invention, I have interposed a thermionic device lll between the tube under test and' meter di. While many types of there micnic devices may be used for the element lill, I prefer a so-called Class B amplifier, i. e., one

i in which the grid bias is approximately equal to the cuteoiic value so that the plate current is approximately Zero when no exciting grid voltage is applied.. In` these devices, plate current ows vduring approximately one-half of each cycle when f an exciting grid voltage of alternating character is present. However, proportionately good results are obtainable to the extent that the output tube approaches these ideal conditions. The tube it is, therefore, illustrated as taking the form of a pliotron and containing a filament Ma, a grid 42 and a plate 43.

The iilament is energized through a circuit M connected to a transformer secondary 145, the mid-point of which is grounded at d6. The grid or electrostatic control element @2 is connected to a variable tap 4l which picks off potentials from a resistor 48. This resistor is connected across the secondary winding 49 of the transformer 33. A connection is taken from a point between the lower end (as shown) of the resistor 48 and winding 9 through a conductor 5I? to the tap 35a. The plate 43 is connected through a conductor 5l to one of the terminals of the measuring instrument di which is preferably a direct current ammeter. The other side of the meter Iii is connected through a conductor 52 to an adjustable tap 53 on the resistor 35.

A consideration of the connections to the .tube fill will show that a voltage of substantially constant magnitude, due to the lter action of the network I9, 2t, is impressed or superposed on the grid llt; This voltage constitutes a grid bias potential, the magnitude ofA which can be. determined by the position of the tap 35a on the resistor 243. The tap 53 on the resistor 35 may be adjusted to provide any desired magnitude of plate voltage applied to the tube Ml.

From the foregoing, it is apparent that the rectifier il and filtering arrangement serve to convert the alternating current voltages applied to the transformer 5 into substantially constant unidirectional voltages which energize in a satisfactory manner the grid and plate circuits of both tubes i and lill. It is also evident that the output of the tube I under test feeds into the input circuit of the combined rectifier and amplier 40, and the output of the latter, in terms of voltage or current variations, is impressed on the meter 4l. The amplier gain of the tube 40 may be adjusted at the tap 41 and this tap also serves as an element for Calibrating the `system in order that the readings on the meter 4I may be simplified, as will be explained hereinafter.

In the operation of the system, as described, a tube having a known dynamic coefficient is substituted for the tube I under test, and the voltages applied to the circuit including the position of all the various taps are adjusted in such a manner that the meter 4I will indicate a reading comparable to the known coeicient of the inserted tube.

The impedance of the circuit, looking to the right from the transformer primary 32, is adjusted by the taps in the input circuit of the tube nl@ so as to provide a substantial matching of the combined tube impedances and the impedance of the meter. Instead of having the meter 4I register the indications in. terms of numerical coefficients, it may be feasible, and in many cases highly desirable, to have the meter register or indicate solely as to Whether the dynamic coeicients of the inserted` tube are good or bad if the inserted tube is known to be a good tube, the meter and the associated circuits are calibrated to indicate this fact. The system is now ready for the measurement of the dynamic coefficients of a tube which is to be tested.

The tube having the unknown `coeiiicients is placed at the position I and, if the circuit has been calibrated accurately, the meter 4I will indicate the desired coelcient either as a numeral or in terms of its quality as good or bad In accordance with another feature of my invention, the potentials impressed on the tube lill are such that the tube passes no plate current when there are no signal potentials impressed on the grid. In other words, the bias on the grid of the tube 4D is so adjusted that the plate current cuts off abruptly when there is no alternating or pulsatory potential applied to its grid. However, when the filtered pulsatory potential is applied to the grid 3 of the tube I under test, the current variatio-ns in the plate circuit of the tube are conveyed through the transformer 33t-ol change the potential applied to the grid i2 of the tube 40.

Under these conditions, the latter will conduct current, the value of which is indicated on the ammeter. tube dll to an abrupt cut-off value, no plate current hows 'through this tube during standby intervals or during the time when nosignal impulses are flowing through the first. tube.

vBy obviating these losses, considerably more accuracy is attained in the system since the current reaching the meter il is determined solely by the signal impulses initiated at the tube under test. In addition to this advantage of reducing current losses, the tube il@ offers the advantage of amplifying the signal voltages so that the meter il need not be as sensitive as in the case of the prior art systems. The fact that only direct current iiows in the plate circuit of the tube iii permits the use of a direct current ammeter of the DArsonval type, which requires only a small amount of energy to operate the same,

since part of the energy for rotating the moving coil is obtained from a permanent magnet within the meter.

It is evident that, byl adjusting the ance matching betweenthe tube I under test and themeterI load which tends to increase the accuracy With which themeter registers or indicates the dynamic coefficients of the tube i. t

Other advantages of the output tube 40 Will readily occur to those skilled in the art and I desire to' comprehendwithin my invention such` modifications of theoutput tubeand associated 1 circuits,`as may be necessary to adapt it to varyying conditions and uses. While it is conceivable l that the tube i under test may ,be of a'special character necessitating a tube li!! ofspecial type,

of grid, amplifying tube, although theClass B xtype is preferred. i It is also` desirable to select for the tube di! av device` having cut-off characteristics which `are flovv Whenthere `are no signal voltages applied tothe grid 62 from the tube under test. Howi ever, it will be understood that these conditions Aare optional, `and the accuracy with which the teristics oi the tube approach theseideal conditions. n i

it will, of course, readily occur to those skilled in the art what changesare necessary in the circuit connections, or rather in the magnitude of the variousresistances and adjustments thereof, to dierentiate between a reading on the mefter Which indicates amplication factor and a reading which iridicatesmutual conductance or any other tube coeicient. The circuit can obviv ously be calibrated `to indicate any one `of these i invention.

readings, either in terms of a number or simply fromthe good-bad quality standpoint.

i It will be understood that Irdesire to compre` hend Within my invention such modifications as come within the scope `of the claims `and the Having thus `fully described myinvention, what claim as new and desire to secure by Letters Patent is:` i i i l. A tube tester for measuring the dynamic 1`cdefi`1cients of a thermionic tube under test hav t ing grid and plateci'rcuits, means including a j source of alternating current, a rectifier thereffor and a filter for supplying direct current potentials to the plate and grid respectively of the] tube under `test but leaving `ripples of sufficient a amplitude in thegrid potential to serve as sig-V "nals, a therrnionic devicehaving input and outto the plate` circuit of the tube under test, and

put circuits, said `input circuit being connected said output circuit being connected to a measuring device, thecombined impedances of the tube `under `test `and of said therrnionic` device including the `connections therebetween substan- H tially matching the impedance of said measuring device. i

2. A`tube tester for measuring the dynamic cuit,an output tube having input and output circuits, said input circuit being connected to the plate circuit of the tube under test, and said output 4circuit being' connected to an electrical ,measuring device, means for applying substantiallyconstant potentials to the plate circuit ofthe tube under test and to the input and ouput circuits of the output tube, both of said means comprising a common source of alternatingcurrent and a rectifier, and` filtering means 1 connected `between said rectifier and the respec` tive tubes for rendering the potential substantially constant but leaving suiiioient pulsations in the gridpotential of the tube undertest` to i serve as signals, the eiiectsof which are `indit cated on the measuring device. abrupt so that positively no plate current will eficiente of a thermionic tube undertest, said tube having grid and"` plate circuits, an output tube having input and output circuits, said in- "put circuits being connected to the plate circuit oi the tube under test and said output` circuit being connected to `a measuring device, the in-` put circuit of said output tube being biased to the extentsuiilcient to prevent current from flowing through the `measuring device except wlienvariable potentialsy are impressed on the igrid circuit o1` the tube under test, a `source 'of alternating current and a rectiiier therefor,"

of said output tube,`and iiltering means inthe l circuits between saidrectiiier and the respective tubes. t

4. A tube tester for measuring the `dynamic col eicients of a thermionic tubeunder test, said tube having grid and plate circuits, an output tube having input and output'circuits, `said input circuit being connected to the plate circuit of the tube under test, and said routput circuit being connected to an electrical measuring device, means for applying substantially constant i potentials respectively to the plate circuit of the tube under test and of the output tube, means for biasing the inputcircuit of theoutput tube Hsuiciently to `cause cut-ofilof the current in the output circuit' except When variable potentials are `applied to the grid circuit of the tube under test,1a source of alternating currentand a rectifier therefor, means for applying voltage from said rectifier to the grid and plate circuits of the tube under test, also to the input and out` put circuits of the output tube andv including the input circuit bias of the output tube.

.. FLOYD E. WENGER. 

